Optical devices such as cameras and integrated camera optics are sometimes integrated into electronic devices such as mobile phones and computers, among others. Manufacturing active and passive optical and electronic components for such devices on a wafer scale is becoming more attractive. One reason is the ongoing trend to reduce the cost of such devices.
Thus, in some applications, the various components are fabricated and assembled on a wafer scale. A wafer scale package, or wafer stack, can include multiple wafers stacked along the smallest wafer dimension (i.e., the axial direction) and attached to one another. The wafer stack may include substantially identical optical or opto-electronic devices arranged side-by-side.
One issue that sometimes occurs during such wafer formation and wafer-scale assembly processes is slight bending (e.g., warping) of the wafers. Such bending may result, for example, because of the relatively small thickness of the wafer or because of the various layers on the wafer, which sometimes are formed in asymmetric arrangements across the surface of the wafer or with different densities on opposite surfaces of the wafer. In some cases, the bending may be as much as 0.5 mm, which reduces the planarity of the wafers and can result in an unacceptable level of non-uniformity in the features across the surface(s) of the wafer. The bending also may adversely impact alignment between wafers when they are stacked one atop the other.